1. Field of the Invention
The present invention relates to optical heads, and more particularly, to an optical head for use in photolithography.
2. Description of Related Art
Along with the development of semiconductor products toward miniaturization, photolithography is continuously required to have a reduced line-width, i.e. a narrower light beam.
Based on Rayleigh's criteria, the resolution (e.g. focal spot) and DOF (depth of focus) depend on the incident wavelength and numerical aperture (NA) of the imaging system. Two methods can be implemented to decrease the focal spot: by reducing the wavelength of the light source and by increasing the numerical aperture (NA) of the lens. The Rayleigh criterion is the generally accepted criterion for the minimum resolvable detail—the imaging process is said to be diffraction-limited when the first diffraction minimum of the image of one source point coincides with the maximum of another. Some attempts have been used to reduce the focal spot by applying near-field optics, for example, a SNOM probe (see “Scanning near-field optical probe with ultrasmall spot size”, L. Novotny, D. W. Pohl, and B. Hecht, Optics Letters, Vol. 20, Issue 9, pp. 970-972 (1995)). A SNOM probe is a light-emitting probe consists of a metal-cladded, sharply pointed transparent structure, most commonly a tapered optical fiber. The foremost end of the SNOM probe is left uncoated to form a narrow aperture. The focal spot of the SNOM probe is mainly determined by the aperture of the SNOM probe and is capable of reducing to several tens of nm (see “Scanning near-field optical probe with ultrasmall spot size”, L. Novotny, D. W. Pohl, and B. Hecht, Optics Letters, Vol. 20, Issue 9, pp. 970-972 (1995)). However, the resulting DOF becomes extremely shallow and causes difficulties in real application.
With a shallow DOF, the exposure environment and platform control will have stricter demands. On the other hand, the shallow DOF may limit the ability to fabricate a structure having a high aspect ratio thereby significantly reducing its implementability.
Therefore, how to provide a deeper DOF for the SNOM probe is becoming one of the most desperate issues in the art.